In the industry, mixtures containing hydrogen and carbon oxides, in particular carbon monoxide, are of considerable importance. The mixtures, usually indicated as synthesis gas, find utilization in a number of well-known commercial processes such as the production of methanol, the synthesis of liquid hydrocarbons and the hydroformylation of olefins.
In the patent and non-patent literature, the said commercial processes have been extensively described and likewise many publications have issued relating to technical features of methods for producing the synthesis gas, to be used as feedstock for one of the aforesaid commercial outlets.
It was realized that the composition of a hydrogen and carbon monoxide containing mixture, suitable to be used as feedstock for one specific outlet, e.g. the production of methanol is not necessarily the same as that of a synthesis gas, intended to be used as starting material for another outlet, such as the production of liquid hydrocarbons.
Accordingly, methods have been developed whereby the process conditions applied in the preparation of a hydrogen and carbon monoxide-containing mixture are modified, for example in order to optimize the hydrogen/carbon monoxide ratio for the intended utilization, or to minimize the content of certain by-products in the mixture which could impede with the further processing thereof.
In EP 112613 various utilizations of hydrogen and carbon oxides containing mixtures are disclosed including the production of methanol, ammonia, synthetic natural gas and normally liquid hydrocarbons. The document in particular relates to the different process conditions and flow schemes which are used in the production of synthesis gas for each of the said utilizations.
As regards the production of liquid hydrocarbons, in the said document an operating scheme is recommended whereby a methane-containing feed is mixed with oxygen, steam and recycled carbon dioxide, the resulting mixture is preheated and is then passed to a reformer comprising a first and a second catalyst zone.
The first catalyst zone contains a partial oxidation catalyst containing palladium and platinum, supported on a honeycomb carrier. The second catalyst zone contains a platinum group metal steam reforming catalyst. The preheat temperature is preferably about 427.degree. to 760.degree. C., the temperature in the first catalyst zone about 954.degree. to 1316.degree. C. and the temperature at the exit of the second catalyst zone 954.degree. C.
The effluent from the second catalyst zone is cooled and passed to a carbon dioxide removal zone. Carbon dioxide is recycled to the feed and the remaining stream is passed to a Fischer-Tropsch hydrocarbon synthesis plant.
For the production of liquid hydrocarbons on a commercial scale the aforesaid operating scheme is not considered attractive.
Large amounts of steam and CO.sub.2 are added to the partial oxidation zone and, hence, at the exit of the second catalyst zone the effluent still contains excessive amounts of carbon dioxide and water. These compounds have to be separated off later in the process, before the reformed product can be used in the Fischer-Tropsch reactor. It would be desirable to be able to produce synthesis gas for use in a Fisher-Tropsch process for the preparation of liquid hydrocarbons without the need to add large amounts of H.sub.2) and/or CO.sub.2 to the process, inter alia in order to prevent soot formation.
Another known process for the production of synthesis gas is disclosed in EP 367654. The process comprises in a first step the partial combustion of a light hydrocarbon feed with oxygen in an amount of at most 50% of the stoichiometric amount required for total combustion, in the presence of steam in an amount less than 1.5 mole per carbon atom in the feed, and in a second step the contact between the combustion gas from the first step and a catalyst containing a Group VI and/or Group VIII metal or compound thereof at a temperature in the range of 800.degree. to 1800.degree. C., preferably of 900.degree. to 1500.degree. C.
It is described in this document that the catalyst in the second step reduces the amount of soot formed in the first step. The catalyst, however, does not substantially alter the composition of the combustion gas from the first step by reforming.
Christensen and Primdahl (Hydrocarbon Processing, March 1994, pages 39-46) describe difficulties and state of the art in autothermal reforming on a commercial scale. It is described in this publication that if it is desired to prepare a synthesis gas having a H.sub.2 /CO molar ratio of about 2, for use as feed for the preparation of synthetic fuels, CO.sub.2 addition is mandatory and the H20 molar ratio should be low, but above 0.5. The CO2/C molar ratio is typically in the range from 0.3 to 0.5
A disadvantage of H.sub.2 O and CO.sub.2 addition is that it gives rise to side-reactions which produce CO.sub.2 and H.sub.2 O respectively. The formation of CO.sub.2 and the addition of is undesired as it gives rise to a non-optimal CO production and, hence, a loss of valuable carbon atoms. Further, CO.sub.2 if present in high amounts in the synthesis gas, has to be separated from the synthesis gas before it can be used in a subsequent process for the preparation of liquid hydrocarbons. In this respect, requirements for a synthesis gas for the preparation of liquid hydrocarbons are different from e.g. requirements for a synthesis gas for the production of methanol. For the latter purpose it is advantageous if the synthesis gas contains an amount of CO.sub.2.
According to Christensen and Primdahl, the H.sub.2 O/C molar feed ratio should be higher than 0.5, typically at least 0.55 or 0.6, inter alia to avoid excessive soot formation.
It would be desirable to be able to operate under such conditions that no detectable soot is present in the synthesis gas and the synthesis gas can be used directly in a process for the preparation of liquid hydrocarbons, without excessive co-production of CO.sub.2 from the synthesis gas.
Further, it would be desirable to be able to operate without excessive use of expensive oxygen and at a high conversion of gaseous hydrocarbon feed.
It has now surprisingly been found that by selecting the temperature of the reformed product stream within the range of 1100.degree. to 1300.degree. C., a suitable feedstock for the production of liquid hydrocarbons is obtained without detectable soot formation, and without requiring high amounts of H.sub.2 O, CO.sub.2 and O.sub.2 addition to the process.